At present automatic transmission systems employ a single oil pump with constant displacement which is driven by the vehicle's combustion engine in proportion to the engine speed. The oil flow at high pressure is used for both lubrication and cooling of the gear train and the torque converter as well as for actuation of various brakes, clutches and shift elements. The constant dependency of the pump operation mode on the combustion engine speed makes this transmission type comparatively inefficient when it comes to the actuation and lubrication losses. For dual clutch transmission systems the lubrication and cooling of the gear train is often realized by an oil-sump based splash-lubrication arrangement. For wet dual clutch transmission systems quite often a volume fraction is taken out of the high pressure actuation circuit and fed to the gear meshes, bearings and synchronizers. All hydraulically actuated dual clutch systems have either a permanently running oil pump or an electrically driven pump.
The transmission systems discussed above have the disadvantage of splash and churning losses due to full or partial submersion of the gears in transmission lubricant fluid. In both automatic planetary-based and dual clutch transmission systems the permanent operation of a pump at a relatively high pressure results in oil leakage which needs to be compensated by respective re-charging. These losses in the actuation and lubrication systems are known as parasitic losses.
In view of the foregoing, it is desirable to overcome or reduce one or more of the above disadvantages. In addition, other desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.